Performing a Tensile Test

Though a tensile test is relatively simple and has been around for a very long time,
some thought and consideration must be done to ensure that the test will have valid
results. Factors involved are the specimen shape and dimensions, the choice of grips
and faces, and many more.

Specimen Shape

The specimen's shape is usually defined by the standard or specification being utilized,
e.g., ASTM E8 or D638. Its shape is important because you want to avoid having a
break or fracture within the area being gripped. So, standards have been developed
to specify the shape of the specimen to ensure the break will occur in the "gage
length" (2 inches are frequently used) by reducing the cross sectional area or diameter
of the specimen throughout the gage length. This has the effect of increasing the
stress in the gage length since stress is inversely proportional to the cross sectional
area under load,
.

Grip and Face Selection

Face and grip selection is a very important factor. By not choosing the correct
set up, your specimen may slip or even break inside the gripped area ("jaw break").
This would lead to invalid results. The faces should cover the entire tab or area
to be gripped. You do not want to use serrated faces when testing materials that
are very ductile. Sometimes covering the serrated faces with masking tape will soften
the bite preventing damage to the specimen.

Specimen Alignment

Vertical alignment of the specimen is an important factor to avoid side loading
or bending moments created in the specimen. Mounting the specimen in the upper grip
assembly first then allowing it to hang freely will help to maintain alignment for
the test.